1. Technical Field
The present invention relates to a metallic separator which is a component part of a solid polymer fuel cell and forms a gas passage for fuel cell, and relates to a production method for the same.
2. Related Art
In a solid polymer fuel cell, a laminated body, in which, on both sides of a planar MEA (Membrane Electrode Assembly), a separator is laminated, is regarded as one unit, and plural units are stacked and form a fuel cell stack. The MEA is formed as a three layer structure in which, between a pair of gas diffusion electrodes that constitute a cathode and an anode, an electrolyte membrane made of, for example, an ion exchange resin, is interposed. In the gas diffusion electrode, outside of an electrode catalyst layer in contact with an electrolyte membrane, a gas diffusion layer is formed. Furthermore, the separator, laminated so as to come into contact with the gas diffusion electrode of the MEA, is provided with a gas passage that allows a gas to flow and a coolant passage between the separator and the gas diffusion electrode. According to such a fuel cell, for example when a hydrogen gas as a fuel is allowed to flow in the gas passage facing the gas diffusion electrode on the anode side, and an oxidizing gas such as oxygen or air is allowed to flow in the gas passage facing the gas diffusion electrode on the cathode side, there occurs an electrochemical reaction, resulting in the generation of electricity.
The separator must function so that, while electrons generated at the anode side according to a catalytic reaction of the hydrogen gas are supplied to an external circuit, electrons from the external circuit may be supplied to the cathode side. Accordingly, for the separator, a conductive material made of a graphite-based material or a metal-based material is used, and in particular the metal-based material is regarded as being advantageous in view of superiority in mechanical strength and in ability to be made lighter and more compact by being formed into a thin plate. As a metallic separator, one in which, for example, a thin plate of stainless steel or titanium alloy, which are each metallic materials having excellent corrosion resistance, is press-molded into a shape having concavities and convexities on the surface, can be cited. In these separators, various techniques are proposed (see, for example, Japanese Patent Application, First Publication, No. 9-22708).
However, when the separator of stainless steel described in the patent document is used, a contact resistance between MEA and the separator is larger than that when a separator of the graphite-based material is used. Increasing the contact resistance induces decreasing performance in power generation. Therefore, gold is coated on the surface of the separator by means of, for example, a coating method, so as to decrease the contact resistance. In the gold coating, a surface treatment is generally performed by nickel coating so as to increase adhesion between stainless steel and the gold. However, when defects, for example, pinholes, are generated in the gold coating, nickel which is a component of the surface treatment is easily eluted. Therefore, there is a problem in the conventional separator that corrosion resistance is low even when the gold coating is performed. Moreover, there is another problem in that the elution of nickel induces decrease of the ion exchange capacity of electrolyte membrane or increase in the peeling of the gold coating, resulting in increased contact resistance.